Volume-rendered images may be useful for representing 3D medical imaging datasets. There are currently many different techniques for generating a volume-rendered image. One such technique, ray-casting, includes traversing a number of rays through the 3D medical imaging dataset. Each volume sample (e.g., voxel) encountered during ray casting is mapped to a color and a transparency value. According to one approach, the color and opacity values are accumulated along each ray using front-to-back or back-to-front volume composition and the accumulated color value is displayed as a pixel in the volume-rendered image. In order to gain an additional sense of the orientation of surfaces within the volumetric data, volume-rendered images may be shaded using gradient shading techniques. Gradient shading techniques compute reflections based on implicitly defined surface normals computed from volume gradients relative to a pre-defined light direction. Both diffuse and specular reflections are taken into account in the gradient shaded volume-rendered image. Other shading methods, such as methods based on computing gradients from a depth buffer may be used instead of gradient shading. Furthermore, volumetric shadowing techniques can be used to enhance perception of depth as well as shapes of structures within the volumetric data. Volumetric shadowing techniques take a predefined light direction or pre-defined light source position into account for computing the shadows. Various methods for shading and volumetric shadowing (hereafter simply referred to as shadowing) are known to those skilled in the art. The shading and shadowing help a viewer to more easily visualize the three-dimensional shape of the object represented by the volume-rendered image.
Some ultrasound imaging systems typically allow the user to control rotation of the volume-rendered image in order to change a viewing direction of the image. However, the resolution of the volume-rendered image may be anisotropic, for example, when the ultrasound image is acquired at fundamental frequencies. As such, the image resolution changes from a radial direction (e.g., a direction normal to the transducer probe surface and in a direction of a probe axis of the transducer probe) to a lateral (e.g., a direction perpendicular to the transducer probe surface normal, also referred to herein as a side view) and elevation direction. For example, when ultrasound data is viewed from the lateral direction, the resulting volume-rendered image has a more noisy and unstable appearance than when the ultrasound data is viewed from the radial direction. Many of the shadows and reflections created in the lateral view volume-rendered image may not correspond to real structures, thereby degrading the ability of the user to make an accurate medical diagnosis. These issues have been recognized by the inventors herein, and are not admitted to be generally known.